1. Field of the Invention
The present invention relates to non-volatile memory devices and, more particularly, to methods of code programming read-only memory (ROM) semiconductor devices.
2. Description of Related Art
A non-volatile, semiconductor memory device is designed to securely hold data even when power is lost or removed from the memory device. The read-only memory (ROM) is a non-volatile memory device widely used in microprocessor-based digital electronic equipment for storing predetermined programs.
Arrays of memory cells are conventionally disposed in ROM devices for storing data, wherein each memory cell includes a transistor. These transistors, which typically comprise metal-oxide-semiconductor field effect transistors (MOSFETs), are disposed near intersecting bit lines and word lines of the memory device. Data bit values or codes held by these memory cell transistors are permanently stored in the physical or electrical properties of the individual memory cells.
A method of programming ROM devices, such as semiconductor devices, which has been explored by the Applicant uses an etch-back spin on glass (SOG) process to expose portions of a photoresist layer located on a substrate, such as a silicon wafer. The etch-back process may be used in forming a pre-code pattern of memory cells on the substrate, which may be used in an attempt to provide larger windows for photo processing. The pre-code pattern can generally be non-selective, and can include all of the potential code positions on the device.
In comparison, a real-code pattern can be formed after the pre-code pattern, and the particular real-code pattern or patterns can be selected based on the desired performance characteristics of the ROM device. To prevent exposure to non-real code pattern positions, such as the code positions that are not elements of the real-code pattern, such a current ROM device processing method explored by the Applicant may utilize wet etched SOG processes. Such a method can be relatively complex and may not provide the desired amount of control in shaping of the processing windows.
Thus, there is a need for improved methods of programming or patterning ROM devices which are simpler and which can provide improved control over spatial aspects of processing windows.